Steam super heater comprising unround pipes

ABSTRACT

The invention relates to a heat exchanger to be used in particular in a waste incineration plant, wherein flue gasses are conducted along pipes through which steam is passed for the production of heated steam. The heat exchanger is comprised of shield pipes ( 8 ) and superheater pipes ( 15 ) provided immediately behind these. The invention also relates to a heat exchanger, whereby flue gasses are conducted along pipes through which steam is passed for the production of heated steam and wherein the heat exchanger is comprised of superheater pipes, which are unround, with their largest diameter in the direction of the flue gas stream.

The present invention relates to a heat exchanger according to thepreamble of claim 1. A heat exchanger of this kind is very usefulespecially, but not exclusively, for the recovery of heat energy fromflue gasses from a waste incineration plant. Although hereafterreference will be made mainly to the use of the heat exchanger in wasteincineration plants, it is also possible to use the same with other hotgasses, especially if they are polluted with dust such as also occurs,for example, when incinerating bio-mass. The invention also relates to amethod of cleaning heat exchanger pipes according to claim 6.

In waste incineration plants it is usual practice to use the hot fluegasses released during incineration of waste for the generation ofsteam. To this end the Waste Fired Power Plant (WFPP) possesses a heatexchanger comprising pipe banks through which steam is conducted thathas to be heated further with the aid of fuel gasses in order to obtainsuperheated steam. For this purpose steam that is formed earlier isconducted via a steam drum as known in the art, through a heat exchangerfor superheating. Such a heat exchanger is generally known as a steamsuperheater. It generally consists of a plurality of sections, eachsection consisting of a plurality of frames, and each frame consistingof two headers between which a plurality of parallel connected pipes areprovided. At the inside, steam or water streams through said pipes andat the outside flue gasses stream over the pipes.

A general drawback of waste is that it contains many pollutants so thatduring incineration flue gasses are released accompanied by much fly ash(dust carried along with flue gasses) and corrosive chemical components(in particular acids such as HCl, SO₂ and a large number of salts). Thisflue gas causes a rapid growth of fly ash deposit on the pipes of theheat exchanger. For the removal of this deposit several techniques areknown in the art. As examples soot blowing, shot peening and rapping thepipes may be mentioned in this context. The chemical components causeserious corrosion, with high-temperature HCl corrosion being the maincause for early pipe wear. This is exacerbated in particular because thegrowing layer of fly ash deposit contains many salts (e.g. manymetal-chlorides) that form eutectics that speed up the corrosionmechanism.

In the present invention the heat exchanger is located in the convectionpart of the boiler, with the heat being transferred directly to thepipes due to the square approach of the flue gasses. The dust in theflue gasses therefore plays an important part in the formation ofdeposit, corrosion and erosion. The dust particles strike the surface ofthe pipes at the velocity of the flue gas carrying them along. Theparticles, which due to the high temperature have become slightlysticky, are able to adhere and accumulate to form large packs ofdeposit. When these packs are removed through cleaning (for example byrapping) and the pipe is stripped once more, the dust particles may alsodamage the metal surface directly before growing into a deposit.Especially pipes made from nickel-chromium alloys, whosecorrosion-protective action consists of an extremely thin oxide-film,this protective layer may be damaged by the cleaning method or byerosion of the clean surface. This causes accelerated corrosion untilthe oxide-film is restored. At this point the effect should be mentionedthat the pipes often do not wear homogeneously, but that they wear thequickest at an angle of 45° to the left and right of the approachingflow of flue gasses.

The NL-1 015 438: “Amsterdam Gem. Dienst: Hoogrendements-AVI” (AmsterdamMunicipal Service: High Efficiency—WFPP) mentions as most importantmeasure the maintenance of low flue gas velocities as method forlimiting the impact of the dust particles, and a low temperature toensure that the material is able to protect itself by forming a newoxide-film. The document also mentions the use of water cooling for thehighly loaded first pipe of a section, to protect said pipe.

An added advantage of the low flue gas velocities is that the smallerparticles will tend to flow around the pipe. This means that inparticular the slightly larger particles collide. Due to their size,these particles fuse less readily with the other particles, with theresult that the growing mass will stay more porous and brittle. This isa great advantage because the growing mass will be able to break off ofthe pipe more easily. In practise a pipe having a diameter of, forexample, 80 mm may exhibit deposits that grow to protrude in thedirection of the flue gas, regularly reaching quite easily up to 300 mmin length. The fore-mentioned cleaning methods endeavour to remove thisdeposit in a boiler in operation, so as to increase the operationalperiod of the boiler between two stops, on which occasion it is cleanedmore thoroughly. Since these cleanings basically involve mechanicalforces exerted on the pack of deposit in order to break the same off,brittleness is a great advantage. Especially with rapping, the pipes ofa frame are caused to vibrate by striking one of the headers with theaid of a hammer mechanism. The cleaning action is in particular focusedon breaking off large pieces caused by the force of inertia experiencedby the mass of the deposited pack when the pipes are vibrating.

It is the object of the invention to provide an improved method andapparatus so as to reduce the fore-mentioned problems of deposit andwear. It is a particular object of the invention to provide an improvedmethod and apparatus for use in a waste incineration plant.

In the present invention an embodiment is proposed that greatly improvesboth the wear of the pipes and the efficiency when cleaning the pipes.To this end the heat exchanger, wherein the flue gasses are conductedalong pipes supplying steam for the production of heated steam, ischaracterized in that the heat exchanger, either entirely or partly,consists of superheater pipes having an unround cross section. This hasa dual purpose: it allows both the flow of the flue gasses around thepipes and the rigidity of the pipes to be influenced.

As already mentioned, the pipes often do not wear homogeneously, butwear quickest at a 45° angle to the left and right of the approachingflue gasses. The choice of the correct diameter ratio in relation to thedistance between the superheated pipes must be based on accurateanalysis, in which the velocities and the mutual distance play animportant role. The diameter is then chosen such that “Von Karmanvortices” (see figure) will not lead to a local increase of the flowvelocities and/or the amount of dust at the surface of the superheaterpipes located in the wake of the pipes in front. This is in contrastwith the typical design methods, where the aim is to use localturbulences to increase the heat transfer.

A simple shape may be obtained by slightly flattening round pipes inorder to obtain unroundness. Other optimised shapes may include an ovalshape, a drop shape or otherwise fluid-dynamically optimised shapes.

The German patent publication DE-444588, by Heinrich Lanz Akt. Ges.“Dampfüberhitzer” (steam superheater) also describes an unround pipe.However, in this embodiment the cross section of the pipe is varied overits length with the object of keeping the flow velocity in the pipeconstant, while due to energy absorption the temperature and the volumeof the steam increases. This is especially relevant when long pipes areinvolved that run zigzag through the boiler. The unroundness is then aconsequence of the proposed method of simply varying the cross sectionof the pipe by rolling it flatter and flatter. In the presentapplication the explicit objective of optimising the flow at the fluegas side is the unroundness. This is applicable also (but notexclusively) to relatively short straight pipes mounted between twoheaders.

Reference is also made to German patent DE 176739 (in the name of W.Fitzner, in 1906) who also describes a heat exchanger with unroundpipes. However, this heat exchanger is used to supply heat by means ofsteam, which therefor cools down. So as to provide for a reduction inthe cross section, the unroundness of the pipes is increased, so as tokeep the throughput of steam more or less constant.

According to a further preferred embodiment the invention relates to aheat exchanger, wherein the heat exchanger comprises several sections,each consisting of superheater pipes that are unround having theirlargest diameter in the direction of the flue gas stream.

Due to the unroundness, the rigidity of the pipes in the two maindirections is clearly different, with the result that the naturalfrequency of the vibrations the pipes are likely to have in thedifferent directions, is also different. Of particular relevance is thatthe natural frequencies in the direction in which a frame is struck bythe rapping device differs from the natural frequencies occurring atright angles thereto. This prevents the applied knocking energy beingdistributed over various directions, which would render it lesseffective.

According to a preferred embodiment, the invention relates to such aheat exchanger wherein the natural frequencies of the unroundsuperheater pipes in different directions, perpendicular to thelongitudinal direction of the pipes, is chosen such that by activatingthe pipe headers the pipes can be made to resonate at differentfrequencies.

In order to determine the fouling, it is possible to determine thecharacteristics of the natural vibrations of the pipes prior to orduring vibrating the pipes for cleaning. To this end the pipe banks maybe struck with an impulse and the resulting vibration image recorded bymeans of instruments measuring said vibrations. These instruments (73)preferably record the movement and/or the forces in the three maindirections. Analysis of these signals, for example, by Fourier analysis,allows the occurring natural frequencies to be determined. Due to thefouling present, the mass of the vibrating pipes is influenced andconsequently also the natural frequencies. By comparing these in variousconditions of fouling, it is possible to accurately calculate thefouling that is present and how the same is distributed over the pipes.The results may be monitored during the cleaning operations andafterwards.

The data available on the basis of the analysis concerning the foulingmay be used to determine that a cleaning needs to be continued for ashorter or longer period of time. Based on these data it is alsopossible to increase or reduce the impulse with which the header of theframe is struck for the purpose of cleaning, in order to obtain optimalcleaning without subjecting the header and pipes to a greater mechanicalstress than necessary or useful.

In addition to the possibility to use an impulse from a knocking deviceit is also possible to make the header vibrate with specificfrequencies. By varying the specific frequency over a large area, thepattern of natural frequencies can be determined with the aid of themeasuring recorders, and the extent of fouling can be derived from that.

This information can be used also for the purpose of cleaning withspecific frequencies that can be varied on the basis of the analysis, soas to make the various pipes vibrate in succession.

As alternative for different stiffnesses in the pipe banks resultingfrom unround pipes it is also possible to influence the naturalfrequencies of the pipe by means of stiffenings and reinforcementsdesigned specifically for that purpose.

The invention further teaches a method of cleaning heat exchanger pipes,comprising the activation of pipes with a previously determinedfrequency in order to make them resonate, causing fouling present on thepipes to come loose.

It is preferred to use such a method in which the previously determinedfrequency is a natural frequency of the pipes.

A further preferred method is one in which the pipes are unround and thenatural frequency depends on the direction of activation, the directionof activation chosen being perpendicular to the longitudinal directionof the pipes.

The invention also relates to a method of determining the fouling on aheat exchanger pipe, wherein the pipe is activated with a previouslydetermined frequency, measuring and analysing the oscillation pattern ofthe pipe, which pattern depends on the degree of fouling of the pipe andthe determination of the degree of fouling, based on the amplitudemeasured.

Preferred is such a method in which the activating frequency is chosenso as to depend on the degree of fouling, in order to obtain the maximumcleaning effects.

According to a further preferred embodiment, the invention relates to aheat exchanger, in which at least two sections are preceded by shieldpipes or evaporator pipes.

According to a further preferred embodiment, the invention relates to aheat exchanger, in which the heat exchanger comprises at least two rowsof extra evaporator pipes, which rows are placed parallel to one anotherand perpendicular to the direction of movement of flue gas, and whereinthe pipes of the individual rows are at least for the main part placedin the path of the flue gas stream.

According to a further preferred embodiment, the invention relates to aheat exchanger wherein the rows of extra evaporator pipes are followedby an open space for levelling out the flue gas stream, which open spaceis larger than the mutual distance of the rows of pipes of the heatexchanger.

The invention will now be further elucidated with reference to thedrawings.

FIG. 1 shows a schematic view of a waste incineration plant, whereinflue gases are conducted from a grate section through a first, secondand third draught, after which the flue gasses are conducted through aheat exchanger to the exit.

FIG. 2 shows a schematic top view of a plant according to FIG. 1.

FIGS. 3-5 show further variants in cross sectional top view of pipes ofa heat exchanger according to the present invention.

FIG. 1 shows a schematic view of a waste incineration plant. The fluegasses are fed to a first draught 1, where they rise vertically and aresubsequently diverted to a second draught 2, where the flue gasses areconducted downward and diverted to a further draught 3. A first draughtis constructed, among other things, from a known membrane wall (notshown).

The flue gasses leaving the third draught are subsequently conducted toa heat exchanger 4 in the form of a steam superheater 5 (in Dutchgenerally indicated as OVO). In the form represented, this OVO comprisesfour different series of heat exchanger pipes 15-18. At the beginning ofthe heat exchanger 4 a so-called evaporator wall 6 is provided. Thisevaporator wall 6 serves to even out the flow of flue gasses approachingthe heat exchanger 5. To this end the evaporator wall 6 preferablycomprises two rows of evaporator pipes, as shown in FIG. 2. After thesetwo rows of evaporator pipes there is preferably a small open space 7,after which one following row of evaporator pipes 8 is provided, afterwhich the first rows of evaporator pipes are placed behind one another,aligned with the pipes of the preceding row of evaporator pipes, as canbe seen in FIG. 2. The small open space 7 is preferably long enough toallow the flue gas velocity to be evened out over the entire flue-flowsurface in this open gap 7 so that its flow velocity is practicallyeverywhere the same.

In the art the evaporator wall 6 is a place subject to fly ash deposit,and rapid cooling of the flue gasses affects the core of the fly ashparticles contained in the flue gasses only after some delay, so thatthey sometimes retain an interior temperature of T>800° C. with theresult that they are still in a so-called “sticky phase”. When theseparticles collide with the successive pipes of the evaporator wall, theywill therefore adhere to its surface. These particles will to aconsiderable degree also adhere to the heat exchanger pipes. In additionto lowering the temperature, moderating the flue gas velocities can alsoreduce this effect. This also results in reduction of the deposit offouling on the heat exchanger pipes. This deposit of ash may be removedfrom the pipes by a method known in the art.

In practice it is preferred for the evaporator wall 6 to even out theflow in order to avoid local high velocities. The flue gas velocity ispreferably 3 to 4 m/s or lower, which results in the surface temperatureof the pipes staying below the flue gas temperature. The evaporatorsection 6 will preferably be provided over the entire width of the gasthroughput at the heat exchanger 4. However, it is possible to reducethe total number of pipes per frame in the evaporator section 6, withthe mutual distance between the pipes being 20-50 cm. If several rows ofevaporator walls 6 are used, it is preferred for the pipes in theindividual rows to be placed in the heat exchanger so as to be staggeredin relation to one another. It is preferred for all the pipes to beequidistanced from one another. In this way there will be an even flowover the height and width of the flue gasses before they enter the OVO15. As there is a free approach to the first row of pipes of the firstOVO, they are preferably embodied as evaporator pipes 8. The remainingpipes of the first OVO 15 are placed behind one another, behind theevaporator pipes B.

The protection of the OVO-pipes by the evaporator pipes is especiallyenhanced if the evaporator pipes are embodied with a diameter that isslightly larger than that of the (viewed in the direction of flow of theflue gasses) succeeding OVO-pipes.

In a further preferred variant, as shown in the FIGS. 3, 4 and 5, theOVO-pipes 41, 30 are embodied slightly oval, with the smallest diameterbeing oriented at right angles to the direction of flow S of the fluegasses. This reduces wear of the pipes 41 resulting from erosion causedby fly ash in the flue gasses. The prior art methods, in which thedeposit 34 of fly ash on the pipes 30 of the OVO as shown in FIG. 4, isremoved by vibrating the OVO-pipes (for example, by striking the headers61, 62, 63 (vide FIG. 5) in which the ends of the pipes are fastenedwith a mechanical or pneumatic hammer 69) may be improved considerablyby giving the OVO-pipes specific natural frequencies that are differentin different directions 71, 72 due to the difference in rigidity causedby the unroundness of the pipes 30. By causing the header 61 to vibratewith these specific frequencies, the deposited fly ash 34 can be removedin a controlled manner. By properly adjusting the natural frequencies ofthe pipes to one another (all the same), a limited energy input willprovide a maximum result. If this is difficult because the adhered massof fly ash deposit is too different, it is also possible to give all theOVO-pipes a different frequency (which moreover is different fordifferent directions of vibration 71, 72) so that it does indeed becomepossible to bring the individual pipes 30 into resonance. A system istherefore preferred, in which the unround pipes have specifically chosennatural frequencies allowing the pipes to be brought into resonance.With further preference a measuring instrument (73) is provided todetermine the oscillation pattern.

In the second section 16 of the heat exchanger or the second OVO 16, theflue gas flow is already distributed evenly, a considerable amount ofdust is already separated and moreover, the flue gas temperature hasbeen lowered. After this first OVO the flue gas velocity may beincreased by either narrowing the boiler, which may occur in steps orgradually, or by increasing the number of pipes per unit of surface. Itis also possible to combine the two embodiments. Depending on the numberof OVOs 15-18 placed in succession in the heat exchanger 4, the same isdesigned such that the flue gas velocity will increase during thefurther progress through the heat exchangers. Special preference goes toin particular the combination of the present invention with a method asdescribed in the European patent application EP-1,164,330, titled “Highefficiency waste incinerator”, possibly in combination with a reductionof flue gas velocity in the flue gas throughput to less than 4 m/s, andpreferably 2 to 3 m/s, and a flue gas velocity through the heatexchanger at the inlet of less than or equal to 4 m/s, at a counterflowoperation of the heat exchanger, wherein the flue gasses at the inlet tothe heat exchanger have a temperature below 700° C., preferably below630° C.

Similarly, the combination with an embodiment described in thesimultaneously filed patent application by the same inventor, whereinsuccessive pipes have different diameters may provide further advantage.

The term “unround” as used in the present specification refers equallyand without limitation also to any substantially round pipe, which isprovided at least over part of its length with reinforcements causingthe pipe to behave like an unround-pipe.

The invention as described above and shown in the figures represents apreferred embodiment of the invention. The invention is limited by theappended claims only.

1. A heat exchanger, wherein flue gasses conducted along superheaterpipes supply heat for the production of heated steam in said superheaterpipes, said superheater pipes having an unround cross section with afirst diameter and a second, relatively larger, diameter and wherein aplurality of pipes are connected to headers, wherein said plurality ofpipes have the same cross section, characterised in that the superheaterpipes are preceded by shield pipes or evaporator pipes (8) that areunround.
 2. A heat exchanger (5) according to claim 1, characterised inthat the unround pipes have their largest diameter in the direction ofthe flue gas stream.
 3. A heat exchanger (5) according to claim 1,characterised in that the natural frequencies of the unround superheaterpipes in different directions, perpendicular to the longitudinaldirection of the pipes, is chosen such that by activating the pipeheaders the pipes can be made to resonate at different frequencies.
 4. Aheat exchanger (5) according to one of the preceding claims,characterised in that one or several shield pipes or evaporator pipes(8) precede the superheater pipes.
 5. A method of cleaning superheaterpipes of a heat exchanger according to any of the claims 1-4, comprisingthe step of activating the pipes with a previously determined frequencyin the direction of the first or second diameter in order to make themresonate, causing fouling present on the pipes to come loose.
 6. Amethod according to claim 5, characterised in that the previouslydetermined frequency is a natural frequency of the pipes.
 7. A methodaccording to claim 5 or 6, characterized in that the natural frequency(or frequencies) of the superheater pipes depends on the direction ofactivation, the direction of activation chosen being perpendicular tothe longitudinal direction of the pipes, for example, in the directionof the flue gas or in the direction perpendicular thereto.
 8. A methodof determining the fouling on a heat exchanger pipe, comprising thesteps of causing the pipe to vibrate, measuring the vibration patterngenerated in the pipe, and determining the degree of fouling, based onthe vibration pattern measured.
 9. A method according to claim 8,characterised in that vibration is caused by an impulse, for example,with a hammer mechanism.
 10. A method according to claim 8 or 9,characterised in that vibration is caused with specific frequencies. 11.A method according to one or several of the claims 8 to 10,characterised in that causing the vibration for cleaning purposes isbased on the vibration pattern measured.
 12. A method according to claim11, characterised in that an activation frequency for causing the pipesto vibrate for cleaning purposes is chosen on the basis of the foulingand natural frequencies determined by the analysis of the vibrationpattern.
 13. A heat exchanger according to one or several of the claims1-4, characterised in that the natural frequencies of the pipes areadjusted by means of stiffenings or couplings.